"While there is a case for allowing some prisoners to stay home overnight as they approach release in order to re-establish community links and aid their transition back into society, overnight release should never be used simply because of a lack of prison cells," shadow justice secretary Dominic Grieve said.

The government insisted the purpose of the move was to rehabilitate offenders, rather than relieve "population pressures" within prisons.

But the Tories are claiming that each night spent by a prisoner out of jail are not counted in prison population figures, helping cloud the real extent of Britain's overcrowding problem.

Each prisoner is only permitted four days' ROR before they are released.